Study Of Shock Wave And Particle Deposition During Cold Gas Spray

Cold gas spray is a relatively new coating technique by which coatings can be formed without significant heating of the sprayed powder. In contrast to the conventional thermal spray processes, such as flame, arc, and plasma spraying, in cold spraying there is no melting of particles prior to impact on the substrate. In cold spray, particles are accelerated to a very high velocity by a flowing gas with supersonic speed and the temperature of spray particles is much lower than its melting point. But as a newly-developed technology, it is short of perfect theory, and the quality and the efficiency of spray should be improved. The supersonic flow of the gas-solid two phase mixture and the deposition process of the particles are two crucial problems during the cold gas spray. The shock wave of gas-solid two phase flow influences the velocity of the particles and the quality of coating. In order to solve these two problems, the following work were done: gas-solid two-phase sound velocity and shock wave,molecular dynamics simulation of the melting and solidification processes of Cu cluster,molecular dynamics simulation of the deposition behavior of Cu cluster on a Cu surface, experiment of cold gas spray and observation of coating by SEM.The supersonic gas-solid two phase flow is used in the cold spray. The sound of two phase flow is quite different from that of single phase flow. Firstly, considering the balance of the two-phase mixture, the isentropic sound velocity of gas-solid two-phase flow was derived, and the sound velocity of different gas-solid two-phase mixture was analyzed. The results show that pressure and volume ratio of gas have give much influence on the sound velocity. This conclusion is consistent with that of Prandtl's analysis and agrees well with the experimental results. Secondly, a two-phase shock wave model based on the two-phase sound velocity was presented. This model is more suitable for the two-phase shock wave than single-phase sound velocity model. The main influence factors on shock wave, such as the phase slip, the heat transfer between phases, different materials and the parameters upstream shock, were analyzed. The results indicate that the phase slip, the pressure of upstream shock wave have a little effects on the shock wave, but Mach number and gas volume ratio of upstream shock wave are the crucial factors. The shock wave formed in front of the substrate decreased the gas velocity greatly. If the sizes of particles are large, the particles would keep high velocity after the shock wave. If the sizes of particles are small, the velocity of the particles would decrease and change the direction. The stable coating is difficult to form in this situation. For the smaller particles, the shock wave should be considered well.During the process of forming coating, the melting and solidification of the particles and the substrate might take place. Using EAM many-body potential function, the melting and solidification processes of Cu clusters which have atoms ranging from 240 to 7164 were simulated by molecular dynamics simulation. According to the changes of some typical parameters, such as potential energy, msd, and the radial distribution function, the melting and solidification points of Cu clusters were obtained, and the influence of the cooling rate on the solidification process was analyzed. It is found that the melting point of Cu cluster is lower than that of bulk Cu. With the increasing of the atoms number, the melting temperature of the cluster increases, but the growth of cluster's melting temperature becomes less and less, and finally the melting temperature of cluster will approach that of the bulk metal. The cooling rate influences the solidification process greatly. Fast cooling brings about the non-crystalline structure, and slow cooling results in the appearance of crystalline metal. The simulation results satisfy the experimental results well.Because it is difficult to observe the deposition process experimentally, deposition processes of single cluster and several clusters on the metal surface were observed by using molecular dynamic simulation. According to the changes of the radial distribution function and the bond pairs, it is found that the crystalline structure of the deposited cluster is similar with that of the substrate. The temperature change of the substrate was calculated. The results indicate that during the impinging process, the melting occurs in the influenced area possibly if the cluster velocity is high enough, but it doesn't melt at the other area of the substrate. The local maximum temperature at the interface, the effective contact area and flattening ratio were calculated to estimate whether the effective combination between the cluster and the substrate is formed. The mechanical and metallurgical bondings are two main ways of combination.Several parameters, such as the potential energy between the substrate and the cluster, the cluster atoms penetrated into the substrate and so on, were used to study the influence factors of deposition process. The flowing influence factors were studied in this thesis: the velocity of the cluster, the temperatures of the substrate and the cluster, the size of the cluster, and the hardness of the substrate and the cluster. Firstly, the incident velocity of the cluster influences the combination strength between the substrate and the cluster greatly. The higher the cluster velocity is, the stronger the combination strength between the substrate and the cluster is, and the faster the cluster forms the crystalline structure like the substrate. Secondly, higher temperature of the substrate and the cluster will improve the combination strength. But the effect of improvement isn't so good. Thirdly, the size of the cluster influences the effect of combination as well. The bigger the cluster is, the stronger the combination strength is. Fourthly, the hardness of the substrate and the cluster is another important influence factor. If a soft cluster impacts on a hard substrate, because of lack of enough transformation at the interface of the substrate, it is difficult to form the effective combination. If a hard cluster impacts on a soft substrate, the lattice defects occur and the cluster takes a longer time to form crystalline structure.Two parameters, the potential energy between the cluster and the substrate and the surface roughness, were applied to study the influence of the size and the velocity on the surface quality. Bigger clusters result in the stronger combination, but the coating is rougher. However, the higher cluster velocity is good for the combination strength and the surface quality.In order to validate the results of MD, the Cu coating on the Al substrate was prepared by experiment of the cold gas spray, and the coating was observed by SEM. Heavy plastic deformation of the particle occurs when the particles impinged with the substrate. The thermal energy, which is tranformed from the kinetic energy of the particles, results in the local melting of the particle. The physical and metal bonds between the particles and the substrate make them combinated tightly. However the small hole formed. Although the conditions between the simulation and the experiment are different, the phenomena observed by the experiment confirm some conclusions obtained by the molecular dynamics simulation.